A papermaking process in a papermaking machine consists of three main parts, namely, forming, press, and drying sections, through which a wet paper web is dewatered continually.
Each section employs papermaking equipments with a dewatering function.
Conventionally, a papermaking felt is employed in the press section, where a felt with a wet paper web thereon is pressurized by a pressing system so that water contained in the wet paper web moves into the felt.
The press portion of the press section is generally composed of a pair of press rolls or a press roll coupled with a shoe shaped to conform to the peripheral surface of the press roll.
Referring to FIG. 1, the structure of the felt is to be described. FIG. 1 illustrates a cross-sectional view of the felt in the cross (CMD) direction. A papermaking felt 10 comprises a base body 20 having batt fiber layers on two sides thereof, a wet paper web side batt fiber layer 31 and a back-side batt fiber layer 32, which are implanted by, for example, needle punching.
The base body 20 is usually a woven fabric made of a warp yarn 21 and a weft yarn 22.
Basic functions of a felt are to dewater a wet paper web (dewatering capability), to improve smoothness of a wet paper web (smoothness), and to transfer a wet paper web (capability to transfer a wet paper web), among which the dewatering function is deemed especially important.
When a wet paper web passes between a pair of press rolls, water moves out from the wet paper web into the felt by pressurization. Water within the felt is either discharged from the underside of the felt or discharged outside of the felt after vacuumed up in a suction box of a papermaking machine. Accordingly, there has been a demand for a felt having a function to be compressed under pressure and rebound when depressurized.
In the field of papermaking techniques, operational speed of papermaking machines and the pressure of a roll or a shoe press in the press section have been increased with an aim to improve productivity. These changes have resulted in a problem that the felt is flattened under high pressure, impairing its water permeability and capability to rebound after compression, which leads to sharp degradation of dewatering capability.
One of the solutions for this problem is to impregnate a fiber layer of a felt with high molecular weight elastic material.
A well-known example is a felt in which fibers are impregnated with emulsion resin and inventiveness lies in a wet paper web side part (U.S. Pat. No. 4,500,588). More specifically, the batt fibers on the surface of the base layer is impregnated with emulsion resin, and a barrier layer is formed on the surface of the wet paper web side of this batt fiber layer by calendaring to make the surface smooth like chamy leather.
However, even the above-mentioned felt, in which batt fibers disposed on the surface of the base layer are impregnated with resin, has left problems unresolved in that it cannot be easily set in a papermaking machine and is not effective enough to prevent rewetting phenomenon in the pressure portion of the press section, when used in recent high-speed papermaking machines, especially in a press section of a closed-draw-type papermaking machine.
Papermaking felts experience rewetting phenomenon in which a wet paper web absorbs water contained in the felt due to negative pressure within the wet paper web produced when the felt is released from pressure at the exit of the press portion of the press section. In a conventional felt (see specification of U.S. Pat. No. 4,500,588), rewetting phenomenon is restrained to a certain extent due to a dense batt fiber layer having resin therein. However, when the whole felt (all the base body and the batt layer) is impregnated with resin, the felt becomes so hardened that the felt cannot be easily set in a papermaking machine. Conventionally, therefore, some felts have resin only in a wet paper web side batt fiber layer. Such a felt, however, becomes incapable of preventing rewetting when used in a press section of a closed draw papermaking machine, because the batt layer contains much water even after pressurization due to low density of the back side batt fiber layer.